1. Field of the Invention
The present invention relates to nonvolatile semiconductor memory having a charge accumulation layer and a manufacturing method for the same.
2. Description of the Related Art
Recently, in conjunction with technical development, centered on flash memories having floating gate structures, nonvolatile semiconductor memory having various structures and constructions have been studied and developed. For example, in view of manufacturing cost reduction, a nonvolatile memory having a floating gate structure formed with a thin film transistor on an insulating substrate made of glass or plastic has been proposed (for example, refer to Japanese Patent Laid-Open Publication No. 87545/1999). In Japanese Patent Laid-Open Publication No. 87545/1999, a nonvolatile memory constructed so that a base insulating layer is formed on an insulating substrate, and on the base insulating layer, a semiconductor layer, an insulating layer, a floating gate (charge accumulation layer), an insulating layer, and a control gate are layered in this order is disclosed.
The nonvolatile memory thus constructed is provided with a charge accumulation layer having a function for holding charges between the control gate and the channel forming region in the semiconductor layer, so that by applying a proper voltage to the control gate and the semiconductor layer, carriers are injected from the semiconductor layer into the charge accumulation layer, or carriers are discharged from the charge accumulation layer to the semiconductor layer. When charges are accumulated in the charge accumulation layer, the threshold voltage of the transistor changes, and accordingly, data storing becomes possible. For example, data is erased by accumulating charges in the charge accumulation layer, and data is written by discharging charges accumulated in the charge accumulation layer.
However, the above-mentioned prior art has the following problems. First, depending on the characteristics of the insulating layer between the semiconductor layer and the charge accumulation layer, when carriers are injected into the charge accumulation layer by applying a voltage to the control gate and the semiconductor layer, the injected carriers are captured in the insulating layer or the quality of the insulating layer changes due to influence from the injection of carriers, resulting in change in threshold voltage of the transistor. If the threshold voltage of the transistor thus changes, threshold voltages in the erasing and writing states are changed, so that the reading margin lowers and discrimination between a writing state and an erasing state becomes difficult.
Second, irradiation of electromagnetic waves or light increases a leak current in the broad sense of the term, resulting in difficulty in discrimination between a writing state and an erasing state.